bundles, making the area likely to be injured if handled roughly; its shape is angulate and the surrounding anatomy can be extremely variable and complex, making this area a high-risk source of cerebrospinal fluid leak during endoscopic endonasal surgery [2]. Keros [3] proposed a classification of the depth of the olfactory fossa into three grades in terms of cranial to caudal length: type 1, 1–3 mm; type 2, 4–7 mm, and type 3, 8 mm or higher, with type 2 being the most common. In addition, the inclination of the vertical lamella can vary remarkably and may also differ from side to side in the same patient. From an endonasal perspective, in case of great depth of the olfactory groove (higher Keros type) and a steep inclination of the vertical lamella, when exposing this area patients are at a higher risk of injury to the skull base during endoscopic procedures (Fig. 7). Similar to the bone, the dura mater that lines the anterior cranial fossa becomes thinner the closer it gets to the olfactory groove, where a dural envelope follows the phyla towards the olfactory mucosa.
Fig. 6. Transfrontal sinus endoscopic view of the median ASB (the trajectory is shown in Fig. 5) before (a) and after (b) section of the outer arachnoid (OAr). Each olfactory bulb (OBu) is enclosed between the crista galli (CGa) medially, cribriform plate (CrP) inferiorly and inferior-laterally, and ethmoidal roof (ER) superior-laterally. Superiorly, each olfactory bulb is connected to the respective gyrus rectus (GR) and medial orbital gyrus (MOG) by a layer of outer arachnoid. The gyrus rectus and medial orbital gyrus of each side are divided by the olfactory sulcus. The medial orbitofrontal artery (MOFA) runs on the inferior surface of the frontal lobe, while the frontopolar artery (FPA) is located within the interhemispheric fissure. OlT, olfactory tract; OR, orbital roof; PS, planum sphenoidale.
Fig. 7. Anatomical variability of the cribriform plate. a Tilted (right side) and high (left side) vertical portions of the cribriform plate. b Non-tilted and low vertical portions of the cribriform plate. Note the asymmetry between the vertical lamellas of different sides in both cases. The white dashed line shows the vertical lamella, and the white dotted line depicts the horizontal lamella. CG, crista galli; NS, nasal septum; T, common lamella of the middle and superior turbinate.
The anatomy of the arachnoid of this region has been described in two different ways. Seeger [4] reported that the olfactory bulbs lie in a subdural area because the outer arachnoid overlies them as a tent; consequently, they are not directly in contact with cerebrospinal fluid. Conversely, Key and Retzius [5] proposed that the arachnoid envelops the olfactory bulbs and phyla following the dura. Thus, the olfactory bulbs are in contact with cerebrospinal fluid. In all likelihood, the truth probably lies somewhere in between: the anatomy of the arachnoid of the olfactory region is variable and shaped by the physiology and hydrodynamics of cerebrospinal fluid, as observed in other skull base areas (i.e., sella turcica).
Fig. 8. Transorbital macroscopic view of the ASB. The orbital roof is formed by the orbital portion of the frontal bone (OPFB) anteriorly and lesser wing of the sphenoid bone (LWSB) posteriorly. The anterior (AEF) and posterior (PEF) ethmoidal foramina lie on the frontoethmoidal suture, between the lamina papyracea (LP) and frontal bone. The white asterisk identifies the optic canal. FR, foramen rotondum; GWSB, greater wing of the sphenoid bone; MT, middle turbinate; NS, nasal septum (perpendicular process of the ethmoid bone); SOF, superior orbital fissure; UP, uncinate process; VC, Vidian canal; Vo, vomer.
The crista galli is a triangular, median process of the ethmoid bone where the falx cerebri inserts anteriorly and inferiorly. It is usually composed of compact bone, but can be pneumatized by surrounding paranasal sinuses in <13% of cases [6]. The foramen caecum lies anterior to the crista galli and can harbor a small vein connected to the superior sagittal sinus. In this area, some nasofrontal dysembriogenic lesions such as dermal sinuses, dermoid cysts, meningoencephaloceles, or nasal gliomas may develop [7]. The anterior portion of the falx cerebri, which extends from the cranial vault to the corpus callosum, divides the anterior cranial fossa into two compartments, which “houses” the frontal lobes and related vessels.
Lateral to the cribriform plate, the midline ASB is formed by the ethmoidal roof, a thick portion of the frontal bone that joins with the ethmoidal box. The basal lamellae of the middle and superior turbinates inserts onto the ethmoidal roof and separates the anterior ethmoid from the posterior ethmoid, and the posterior ethmoid from the sphenoethmoidal recess, respectively. In the area where the lamina papyracea joins with the ethmoidal roof, the skull base tilts superiorly as a consequence of the convex shape of the orbit. The dihedral angle where the skull base turns from horizontal (ethmoidal roof) to convex (orbital roof), called the orbital beak, can be pneumatized by the superior-lateral extension of a suprabullar cell (supraorbital ethmoid cell) or the posterior extension of the frontal sinus (supraorbital recess of the frontal sinus). The ethmoidal arteries are collateral branches of the ophthalmic artery and enter the ethmoidal foramina located along the suture between the lamina papyracea and ethmoidal roof (Fig. 8, 9). In the majority of cases two arteries are found, one per ethmoidal compartment (anterior and posterior). In addition, a middle ethmoidal artery has been reported in 29–38% of cases [8–10]. Anterior ethmoidal arteries usually follow a caudal course with respect to the ethmoidal roof, while the middle and posterior ethmoidal arteries usually run into the skull base. The ethmoidal bony canals can show focal or wide dehiscent areas. The ethmoidal arteries divide into several small arteries for the nasal septum, middle and superior turbinate, and external nose, while the terminal branch perforates the vertical portion of the cribriform plate, forming a bony defect called ethmoidal sulcus, and contributes to the vascularization of the dura of the anterior cranial fossa (anterior meningeal arteries). Because of their relevance in ASB surgery, the anterior meningeal arteries running within the falx cerebri along the anterior surface of the crista galli have been specifically called anterior falcine arteries [1]. The skull base anterior to the ethmoidal roofs and cribriform plates is composed of the posterior plates of the frontal sinuses, which tilt from coronal to axial above the frontal recesses, constituting the transition area from cranial vault and cranial base.
Fig. 9. Transorbital endoscopic view of the ASB via a superior eyelid approach. The endoscope is placed in a posterior-medial (a), posterior (b), and posterior-lateral (c) direction through the superior orbital quadrant, between the orbital roof and periorbit (Per). d The yellow, green, and red circles show the areas targeted in a, b, and c, respectively. Several landmarks can be identified from anterior-medial to posterior-lateral: the anterior (AEF) and posterior (PEF) ethmoidal foramen along the frontoethmoidal suture (FES), the optic canal (OC), the superior orbital fissure (SOF), and Hyrtl’s foramen (HF), when present. GWSB, greater wing of the sphenoid bone; OPFB, orbital plate of the frontal bone; LWSB, lesser wing of the sphenoid
